I am in the same situation. The only difference is my current contract expires in May. But just like other blackberry addicts, I dont want to wait. Also, from what I hear, even if the 8700 will be available as an upgrade, there wont be much of a price drop. So I'm thinking it would cost me at least 300.

I stumbled across a retailer site ushopwireless and they're offering preorders for NEW accounts only. They offer it for $99 after a $100 rebate.
I called T-Mobile and they said that one person can have 2 separate accounts open. I plan to use my current Sim card until the contract expires next month. And then I can cancel my current service. The only issue is the phone number porting. The T-Mobile rep told me that I wont be able to port since its within the same provider and I'd have to keep that number (or get a new one). If I really am not able to port, since I really need to keep my number, I plan to go to another provider (maybe AT&T), create new service, and port my number there. Once my number is ported, I should be able to port it back into T-Mobile, then I can cancel the AT&T as well as my current T-Mobile service.
I know its a pain, but I think this is the only solution to upgrade and save a few bucks. Any opinions on this?? Thanks. Good luck.

Is that place taking preorders reputable? Sounds kinda risky to me. I bet Amazon.com will do the deal they had for the 8700c on cingular. Free at one point and now just rebates to $100 which is very reasonable.